1. Field of the Invention
The present invention relates to bidirectional switches controllable by a low-voltage signal. The present invention more specifically relates to bidirectional switches intended for being connected in series with a load supplied by a high A.C. voltage, for example the mains and, among such switches, those that are controllable to be turned off and on.
2. Discussion of the Related Art
Switches for A.C. loads of the type to which the present invention relates are used, in particular, each time an interface circuit between a low-voltage control system, for example a microcontroller, and a load to be supplied by an A.C. high voltage, for example, an engine, a heating resistor, etc., is required.
An example of application of the present invention relates to electric household appliances, for example, washing machines, in which several loads (engines, pumps, heating resistors, valves) are to be controlled from the machine microcontroller.
Generally, a bidirectional switch adapted to being controlled by a low-voltage signal is formed of a triac in series with the load to be controlled and the gate of which receives a low-voltage control signal.
A problem that arises with a triac is that it must be started for each halfwave of the supply voltage since it blocks (is non-conducting) when the current disappears between its two power terminals. Accordingly, in conventional systems, a D.C. signal is often applied on the triac gate for the entire duration when it must be on.
A disadvantage is that this D.C. signal, generally provided by a microcontroller that draws its power from a low-voltage power supply, results in continuous power consumption.
To avoid using a D.C. signal, a pulsed control is provided when one or several triacs are meant to be controlled by a microcontroller. The control is then performed by pulses upon each change of biasing of the A.C. power supply. These pulses, present during the entire closing time, monopolize the microcontroller. Further, for small loads, it is generally not possible to control the triac with a pulsed signal at the mains frequency. Indeed, the triac hold and starting cur-rents then have the same order of magnitude (some ten milliamperes) as the current in the load.
Another disadvantage is that the supply circuit of the microcontroller or the like then dissipates significant power, since a voltage conversion from a high A.C. supply voltage is most often used.
It would be desirable to have a high-voltage bidirectional switch that can be controlled by a low-voltage signal while reducing or minimizing the power consumption and the use of the possible control microcontroller.
Another disadvantage of the triac is that it provides no information about its state (on or off). Now, it can be desirable to known this state, for example, to detect a parasitic triggering.
Thus, an object of the present invention is to provide a bidirectional flip-flop capable of being controlled by an irregular pulsed signal, that is, only providing one pulse for the turning-on and one pulse for the turning-off.
The present invention also aims at providing a bidirectional switching circuit respecting the structure of a triac, that is, including two power terminals and one control terminal. Thus, another object of the present invention is to enable control of the switch by a pulsed signal applied on a single terminal of the circuit.
The present invention also aims at providing an autonomous circuit, that is, a circuit requiring no external low-voltage power supply for its operation.
The present invention also aims at enabling detection, by the control circuit, of the state of the switch.
The present invention also aims at providing, in a simple way, an information about the state (off or on) of the switch.
The present invention further aims at providing a bidirectional switch that is entirely integrable.
To achieve these and other objects, the present invention provides a bidirectional switch, including a first bidirectional switching means between two power terminals of the switch, a low-voltage storage element between a first power terminal and a control terminal of the switch, and a control stage adapted to organize, upon each halfwave beginning of an A.C. supply voltage applied between the power terminals and when the switch is on, the charge of the storage element with a biasing depending on the sign of the halfwave.
According to an embodiment of the present invention, the turning-on of the switch is caused by the occurrence of a low-voltage pulse of charge of the storage element on said control terminal.
According to an embodiment of the present invention, the turning-off of the switch is caused by forcing the discharge of the storage element, so that the first bidirectional switching means remains blocked at the beginning of the following halfwave of the A.C. supply voltage.
According to an embodiment of the present invention, the switch includes a means for delaying the turning-on of the first bidirectional switching means with respect to the zero crossing of the A.C. supply voltage, to leave the storage element time to charge.
According to an embodiment of the present invention, the control stage includes a second bidirectional switching means between said control terminal and a second power terminal of the switch.
According to an embodiment of the present invention, the control stage includes at least one switch connecting the control terminal of the second switching means to the first power terminal of the switch.
According to an embodiment of the present invention, said second bidirectional switching means is formed of two control thyristors connected in antiparallel and the respective gates of which are individually connected to a first terminal of application of the A.C. supply voltage via a switch of the control stage.
According to an embodiment of the present invention, said second bidirectional switching means is a triac having its gate connected to a first terminal of application of the A.C. supply voltage via a switch of the control stage.
According to an embodiment of the present invention, said first bidirectional switching means is formed of two one-way switching elements connected in antiparallel between the two power terminals of the switch.
According to an embodiment of the present invention, said switching elements are formed by power thyristors respectively with a cathode and an anode gate, the gates of said power thyristors being individually connected to the control terminal of the switch via a Zener diode.
According to an embodiment of the present invention, each power thyristor is associated with a control thyristor having the same type of gate.
According to an embodiment of the present invention, said first bidirectional switching means is a triac having its gate connected, via two Zener diodes connected head-to-tail, to the control terminal.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.